Speckle is an observable random intensity pattern produced when a coherent light beam is diffusely reflected from a target, and it is a fundamental source of noise in optical measurement systems. It results from the interference of different reflected portions of an incident beam having random relative optical phases, and is in part dependent on the profile of the reflective surface.
Active optical heterodyne detection systems may be used for stand-off detection of small localised absorption or dispersion signals. It is an object of the present invention to provide an effective method for reducing speckle noise in optical measurement systems and in particular in active optical heterodyne detection systems.